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Ancient humans’ ability to walk upright may have first evolved on trees before they took to the land, according to a new study that analysed the behaviour of chimpanzees.
These findings, published in the journal Science Advances, challenge an older theory suggesting that our ancestors first evolved to walk upright on the ground in Africa as the landscape changed to an open, dry savanna environment with fewer trees.
“Our study suggests that the retreat of forests in the late Miocene-Pliocene era around five million years ago and the more open savanna habitats were, in fact, not a catalyst for the evolution of bipedalism (walking upright),” Alex Piel from University College London and co-author of the study, said in a statement.
Trees probably remained essential to the evolution of this trait, he added.
This shift necessarily shaped our anatomy from quadrupedalism (walking on all fours) to bipedalism, Piel told Down To Earth. The pelvis’ shape and the leg bones’ angle are some of the anatomical changes that came about.
Piel and his colleagues arrived at these findings by analysing the behaviour of chimpanzees, which are closely related to humans. The experts studied the primates for 15 months in Western Tanzania, where the landscape is similar to that of ancient humans.
Fossil records have revealed that early humans showed apelike and humanlike ways of moving around between six and three million years ago, according to the Smithsonian National Museum of Natural History.
Though fossil bones can be a rich source of information, they have limitations. They don’t do a good job of reconstructing the behaviour of early humans, the researchers argued in the study.
So they turned to chimpanzees, whose last common ancestry (LCA) with humans dates to roughly 5-7 million years ago. Piel and his team monitored 13 chimpanzee adults, including six females and seven males.
They documented almost 2,850 observations of the animals climbing, walking, hanging and the like. The researchers also noted whether they walked upright on the ground or in trees.
They analysed the data to investigate a link between tree or land-based behaviour and the type of vegetation, meaning forest vs woodland. Chimpanzees were more tree-bound despite the limited tree cover. The time spent on trees was similar to chimpanzees from dense forests, the study showed.
The researchers don’t know why bipedalism began on trees instead of land. But they have some theories.
Humans evolved in landscapes similar to Issa Valley, where threats from predators such as lions, leopards, wild dogs and hyenas were high. “It would certainly be safer in trees and there may be larger, more abundant food patches that allow them to remain arboreal for longer periods,” Piel said.
Another 2007 study published in Science also theorised that bipedalism began on trees after studying orangutans. The LCA between humans and orangutans was roughly from 20 million years ago, Piel said.
The 2007 study also theorised that bipedalism was likely driven by the need to reach high-hanging fruits in trees.
This may have driven the split between apes who remained quadrupeds and those that stood up to reach those fruits, Piel explained.
The researchers now hope to look for clues to explain the origin of this trait in humans. “We would also like to look into the interaction between seasonality, food distribution, and predation risk,” he highlighted.